Semiconductor fabrication typically requires fabricating multiple layers on a structure in which some or all of the layers include fabricated features. Overlay metrology is the measurement of the relative positions of structures on various layers of a sample, which are critical to the performance of a fabricated device and must typically be controlled within tight tolerances. For example, overlay metrology may measure the relative positions of features on different sample layers as a measure of the layer-by-layer alignment of fabrication tools. By way of another example, overlay metrology may measure the relative positions of features on the same layer as a measure of the alignment of multiple exposure steps on the sample layer.
Not all device feature layouts are amenable to direct overlay measurements. Further, overlay measurements may damage or otherwise affect the performance of device features. Accordingly, overlay measurements are commonly performed on dedicated overlay targets having features designed for sensitive overlay measurements rather than directly on device features. However, differences in size, orientation, density, and/or location on the sample of overlay targets relative to the device features may introduce a mismatch between measured overlay at the target and actual overlay of device features. Accordingly, ensuring device-relevant overlay measurements on overlay targets remains an ongoing challenge in overlay metrology.
Further, efforts to achieve device-relevant overlay measurements must typically be balanced with throughput requirements. For example, overlay targets with device-scale features may provide device-relevant overlay. However, device-scale features are typically resolvable using particle-beam metrology tools such as, but not limited to, scanning electron microscopes (SEMs) that may limit throughput in a production environment. In contrast, optical overlay metrology may provide higher throughput, but may require overlay target features substantially larger than device features and may thus be more susceptible to errors.
It is therefore desirable to provide systems and methods for overlay metrology that provide device-relevant overlay accuracy while balancing throughput requirements.